1. Field of the Invention
The invention relates in general to digital signal broadcasting technologies, and more particularly, to an apparatus and a method for estimating channel effects of Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) signals.
2. Description of the Related Art
Developments of digital television broadcasting have gradually matured with progresses of communication technologies. Besides being transmitted by cables, digital television signals may also be transmitted in form of wireless signals via base stations or artificial satellites. Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) is a prevalent standard adopted in the field of digital television broadcasting. However, during a transmission process, wireless signals are inevitably affected and interfered by transmission environments. Therefore, in order to correctly parse received data, an ISDB-T receiver needs to first evaluate corresponding channel effects and thus eliminate influences posed by the channel effects upon signal contents.
Each data frame in an ISDB-T signal includes 204 orthogonal frequency division multiplexing (OFDM) symbols, each of which includes contents bits carried by multiple subcarriers. FIG. 1 shows an example of a content configuration of an ISDB-T signal, with the horizontal axis representing the frequency and the vertical axis representing the symbol number (the greater the number, the later the transmission time). As shown in FIG. 1, subcarriers at frequency indices in multiples of 3 (e.g., 0, 3, 6, 9 . . . ) carry a scatter pilot (SP) signal at every other four symbols.
Original contents of the above SP signal are known to a receiver, and may thus serve as reference for the receiver to determine channel effects. For example, an ISDB-T receiver may first identify a frequency-domain channel effect H(t=0, f=0) of the SP signal having a time index of 0 and a frequency index of 0, and a frequency-domain channel effect H(t=4, f=0) of the SP signal having a time index of 4 and a frequency index of 0. With time-domain interpolation, the ISDB-T receiver determines frequency-domain channel effects H(t=1, f=0), H(t=2, f=0) and H(t=3, f=0) between H(t=0, f=0) and H(t=0, f=0). Similarly, the ISDB-T receiver may also obtain frequency-domain channel effects H(t=2, f=3), H(t=3, f=3) and H(t=4, f=3) between H(t=1, f=3) and H(t=5, f=3) through interpolation.
In general, the ISDB-T receiver determines time-domain interpolation coefficients according to the ratio of time intervals. For example, it is determined H(t=1, f=0)=H(t=0, f=0)*0.75+H(t=4, f=0)*0.25; that is, the time-domain interpolation coefficients corresponding to H(t=1, f=0) are determined to be 0.75 and 0.25. Similarly, H(t=2, f=0)=H(t=0, f=0)*0.5+H(t=4, f=0)*0.5, and H(t=3, f=0)=H(t=0, f=0)*0.25+H(t=4, f=0)*0.75. In a situation where channel effects rapidly change as time progresses (e.g., in a propagation environment containing Doppler effects), the above method for selecting time-domain interpolation coefficients is likely to result in incorrect interpolation results.